It is desirable that when liquid is drawn into a hypodermic syringe barrel for subsequent injection into a patient the liquid be filtered to remove stray solids, e.g. rubber from a cap through which a needle is inserted for the removal of liquid from a container, and it is also desirable that the liquid, before it is expelled from the syringe barrel, be filtered again. Obviously different filter areas should be used for the entry and exit of liquid into and out of the syringe.
The recognition of the desirability of presenting fresh filter areas to fluid as it passes into and out of a syringe barrel is embraced in the patent to Cohen U.S. Pat. No. 4,137,917. In that patent a continuous sheet of filter material is attached to an apertured slider having a pusher handle extending outwardly of a housing having an apertured socket on one side to receive the usual needle hub of the syringe and an apertured nipple on the other side which is a duplicate of the usual needle hub on the syringe. As the slider is slid into the housing, successive apertures in the slider, aligned with separate filter areas of the sheet of filter material, are brought into alignment with the openings through the hubs on the syringe and housing and thus the fluid is caused to pass through fresh filter areas as fluid is pumped into and out of the syringe barrel.
There are a number of problems with such an arrangement. For example, a continuous layer of filter material is used and it is clear that liquid passing through one area of the filter material will naturally flow by capillary action laterally through all of the filter material. Thus the entire layer of filter material can be contaminated by infectious organisms. Further, the handle which must be slid from outside the housing into the housing can be a source of infection, and, finally, the housing itself constitutes an awkward visual and physical obstruction.